The permanent mammalian myogenic cell lines, L6 and L8, can be induced to undergo differentiation from presumptive myoblasts to myotubes. These cells, therefore, provide a well defined biological system in which to study the control of specific gene expression during a specific developmental program. We propose to identify and quantitate the interacting levels, transcriptional and post-transcriptional, of gene regulation that are involved in cell differentiation. These experiments will utilize cDNA clones and genomic DNA clones specific for alpha-, beta-, and gamma-actin, and acetylcholine receptor alpha-subunit mRNAs. The expression of beta- and gamma-actins are repressed during myogenic differentiation, while the two other genes appear to be coordinately activated during this developmental program. The cloned DNA sequences will be used to determine the rates of transcription, the kinetics of formation of nuclear precursors, nucleocytoplasmic transport, and cytoplasmic turnover of these mRNAs. A detailed structural analysis of these transcriptional units will be related to common features in the metabolism of their gene products. The combination of metabolic and structural approaches will provide the basis for molecular studies on myogenic cell mutants and neuromuscular morphogenesis, and will contribute to an understanding of the regulation of gene expression during development.